The present invention relates generally to thin film photovoltaic devices and in particular the invention provides a structure and a method of forming the structure for thin film cells to achieve light trapping in these cells.
The use of light trapping is well know in monocrystalline silicon cells where light trapping features in the surface of the cell have dimensions which are much less than the thickness of silicon substrate of the device and significantly greater than the wavelength of light in air.
In solar cells, light scattering is used to trap light in the active region of the cell. The more light that is trapped in the cell, the higher the photocurent that can be generated which leads to higher efficiency. Therefore, light trapping is an important issue when trying to improve efficiency in solar cells and is particularly important in thin film cell design.
However it has been widely thought that in thin film devices where the active silicon layers are thin films formed over a substrate such as glass, light trapping would not be possible or at least demonstrate reduced effectiveness. This is because the film thicknesses are of the same order of magnitude or thinner than the dimension of light trapping features in known monocrystalline devices. As film thicknesses in thin film devices are reduced, they tend toward conformal coatings having predominantly parallel surfaces over the etched surface of the glass substrate and conventional thinking would be that such an arrangement does not achieve significant advantages from light trapping. Also as film thicknesses are reduced to the order of a wavelength (in air) or thinner conventional thinking is that the mechanisms providing light trapping in prior art devices would cease to be effective.
This is born out in prior art thin film amorphous silicon solar cell devices where no deliberate attempt at texturing was made. Present day amorphous silicon devices typically comprise a glass superstrate over which is layed a Transparent Conductive Oxide (TCO) contact layer and a thin amorphous silicon film (1 xcexcm) active layer including a p-n junction and a rear metallic layer acting as a reflector and back contact. When such structures were first devised it was noticed that in some circumstances (when the surface of the TCO was cloudy) cell performance was greater than expected but the literature offered no explanation as to the reason for such unexpected performance.
It has now become apparent to the present inventors that light trapping is possible in thin film devices and the prior art amorphous silicon devices were in fact demonstrating a characteristic that the present inventors have now identified and adapted to crystalline silicon thin film cells. Central to the invention is the realisation that the wavelength of light of a given frequency is different in silicon and air.
The present inventors have now identified the circumstances under which light trapping can be achieved in thin films and in particular the inventors have devised methods for manufacture of thin film crystalline silicon solar cell structures which exhibit light trapping characteristics.
The methods of achieving light trapping devised by the present inventors generally involve texturing a surface of the substrate on which the thin film is formed. Conventionally, glass textures are made by chemical texturing and sand blasting. Recently, metal crystal deposits have been used on a substrate surface to form very fine crystals to produce texture effect.
However, both chemical texturing and sand blasting cause cracks and non-uniform feature size on the glass surface, each of which can adversely affect solar cell fabrication and/or performance, such as by causing shunting in the devices. It is assumed that it is for this reason that there are apparently no reports of high efficient solar cell fabrication being achieved on either chemical textured or sandblasted substrates. Furthermore, the method used to perform the chemical texturing also produces waste products that are environmentally hazardous and therefore represent a severe pollution risk. On the other hand, the use of fine metal crystals to form a textured surface is an expensive approach and significantly adds to the cost of solar cells manufactured using this technique.
According to a first aspect, the present invention provides a method of forming a light trapping structure in a thin film silicon solar cell formed on a glass substrate or superstrate, the method including the steps of:
a) providing a textured surface on the glass substrate or superstrate, the texturing layer comprising texturing particles held in a binding matrix; and
b) forming a silicon film on the textured surface and forming a photovoltaic device structure in the silicon film, the silicon film being less than 10 xcexcm thick.
According to a second aspect, the present invention provides a thin film photovoltaic device incorporating a light trapping structure, wherein the photovoltaic device is formed on a textured surface provided on a glass substrate or superstrate, the photovoltaic device comprising a thin silicon film into which is formed at least one pn photovoltaic junction, the silicon film being less than 10 xcexcm thick and the textured surface being provided by a texturing layer over a surface of the substrate or superstrate and comprising texturing particles held in a binding matrix.
The invention is applicable to both crystalline and amorphous silicon solar cells. In the case of amorphous silicon cells, a thin silicon film is formed over a TCO layer, or alternatively the texturing film containing the texturing particles may itself be of TCO material.
In embodiments of the invention, the texturing layer preferably includes surface features having dimensions in a range from 0.5-0.2 times the thickness of the silicon film.
The silicon film is typically less than 5 xcexcm thick and preferably has a thickness of 2 xcexcm or less. The silicon film is typically at least 0.5 xcexcm or greater and preferably greater than 1 xcexcm. Typically, the scale of textured surface features is in the range of 0.01-10 xcexcm. The useful lower limit of the feature size is in the order of a wavelength of light in crystalline silicon and typically the useful lower limit is 0.05 xcexcm. The texturing may also include large scale features which have dimensions greater than the thickness of the silicon film.
In one set of embodiments, a textured layer is applied to the glass surface by mixing crushed quartz having particle dimensions in the order of 0.5-3 xcexcm and preferably in the order of 1-2 xcexcm into a glass sol, applying the mixture to the glass surface and heating to sinter the glass sol to form a dielectric layer. Note that the final feature dimension of the surface in this case includes quite small features, being determined largely by the surface roughness of the crushed quartz and the thickness of the dielectric layer, as well as the particle size of the quartz.
In preferred embodiments of the invention, the texturing of the substrate or superstrate is achieved by an SiO2 layer which includes monospheric SiO2 particles in the range of 0.1-2 xcexcm in diameter. Preferably, the monospherical particles are in the range of 0.5-0.91 xcexcm and in a particularly preferred embodiment, the particles are approximately 0.7 xcexcm (eg, 0.65-0.75 xcexcm). The particles are located in a smooth SiO2 film having a thickness in the range of 0.2-0.8 times the diameter of the monospheric particles. Preferably, the SiO2 layer is in the range of 0.35-0.5 times the particle diameter and in the particularly preferred embodiment, is approximately 0.3 xcexcm (eg, 0.25-0.35 xcexcm). The difference between particle dimension and film thickness results in a textured surface, however in this case the features can be more greatly spaced as they are larger. Both the SiO2 particles and the film are made by a Sol-Gel process.
The SiO2 layer also provides a barrier layer between the glass substrate and the silicon film. A separate barrier layer may also act as an anti-reflection layer and will be arranged to have a thickness equal to a quarter wavelength xc2x1 20% which in the case of silicon nitride is 70 nm xc2x120%.
In typical embodiments of the invention the back surface of the silicon film structure (ie, remote from the glass) has a reflective material formed over it. Typically, the reflective material will be a metallisation structure used to contact the active regions of the cell. The metallisation structure will in some embodiments, be separated from most of the silicon back surface by an insulating layer.